DN50x40 PN20 A105 Two-Piece Extended Stem Floating Ball Valve, Reduced Bore, RF Flanged

What a DN50x40 PN20 A105 Extended Stem Floating Ball Valve Is

A DN50x40 PN20 A105 extended stem floating ball valve is a two-piece, reduced bore, soft-seated isolation valve. The ends are DN50 (2 inch) raised face flanges, the bore through the ball is DN40 (38 mm), the body is forged ASTM A105 carbon steel in the normalized condition, the pressure class is PN20 / ASME Class 150, and the stem is extended beyond the body so the packing gland and the operator sit clear of insulation, burial depth or a hot surface. The design basis is API 608 with ASME B16.34, face to face is 178 mm to ASME B16.10, and testing is to API 598.

That sentence contains five separate purchasing decisions, and almost every wrong order for this valve comes from collapsing them into one part number. DN50x40 is not a size, it is an end size plus a bore. PN20 is not a flange standard. A105 is not a low temperature material. Extended stem is not a fixed length. And floating is a sealing mechanism with hard limits, not a cheaper version of something else. The sections below take each decision in turn, and say where this build stops.

The valve suits insulated steam and condensate branches, heat traced process lines, buried yard piping and skid packages where the pipe is 2 inch but the design flow only needs 1.5 inch. It is not a substitute for a trunnion mounted ball valve at high pressure, it cannot provide double block and bleed, and it stops at -29 C on the cold side and at the seat temperature on the hot side.

Decoding the Model String

A model string is written by the seller and read by the buyer, and the two rarely mean the same thing. Each token below fixes one property and leaves one question open. The right hand column is what to confirm in writing before the order is placed.

Read this way, the model string stops being a part number and becomes a checklist. The two tokens that cause the most field problems are PN20, because it is a pressure class disguised as a flange standard, and the extended stem length, because it is the only dimension on the valve that depends on how the line is built around it rather than on the valve itself.

Why Reduced Bore: DN50 Ends on a DN40 Bore

The reduced bore is the one feature on this valve that cannot be changed later, so it deserves an explicit decision. DN50 ends let the valve drop into a 2 inch line without a reducer, a spool piece or a change of gasket size. The 38 mm bore inside sets the flow capacity, the operating torque, the weight, the price and whether the line can ever be pigged. Those are separate arguments and they usually point the same way on small insulated lines, which is why this configuration exists.

The bore is not a restriction to be apologised for. On a branch line sized by a future expansion, or on a line whose flow is controlled downstream by a valve or an orifice plate, the 1.5 inch bore passes the design flow with a pressure drop measured in hundredths of a bar. What it costs is velocity: inside the bore the fluid speed rises by the square of the diameter ratio, so it is 1.73 times the line velocity. For water at 2 m/s in the pipe, that is 3.5 m/s in the bore, which is ordinary. For a gas line carrying sand, or a slurry, 1.73 times is enough to change the erosion life of the seat and the ball, and that is the point at which a full bore valve stops being optional.

Accept the reduced bore when the line is sized for a future duty, when flow is trimmed downstream, when the valve is hand operated at full differential pressure and torque matters, or when weight and cost are counted per unit on a skid with dozens of valves. Refuse it when the line is pigged, when the fluid carries solids at speed, or when the bore is being used to control velocity rather than to pass flow. A reduced bore valve in a pigged line is not a compromise, it is a blockage waiting for the first cleaning run.

How a Floating Ball Seals, and Why It Needs No Cavity Relief

In a floating ball valve the ball is not carried on a shaft. It rests between two seat rings with a small clearance, and line pressure pushes it downstream until it bears on the downstream seat. Sealing happens in four steps. Pressure enters the upstream half of the body and acts on the exposed area of the ball. The ball moves a fraction of a millimetre downstream. The downstream seat, which is preloaded by its own spring or by the seat carrier, is compressed between the ball and the body. The contact stress at that interface, held by pressure rather than by a mechanism, is what stops the leak.

Because the ball has to move to seal, the upstream seat never seals against pressure. It is pushed away from the ball, not into it. The body cavity therefore stays connected to the upstream line at all times, which has two consequences that are worth stating plainly. First, nothing is trapped in the cavity, so thermal expansion of the fluid cannot overpressure the body and no cavity relief valve is needed. Second, and less convenient, the valve cannot isolate its own cavity, so it cannot provide double block and bleed. If the duty requires DBB, or a positive double block for maintenance, this is the wrong product and a trunnion mounted valve with two piston effect seats is the right one.

The same geometry sets the pressure limit. Seat load is pressure times seat area, and the whole of that load passes through the seat rings rather than into bearings, so friction and torque climb steeply with bore. At DN50 and PN20 the numbers are small enough for a lever. At DN150 and Class 300 the same design would need a gear operator and would wear its seats out early, which is exactly where the trunnion range begins.

Two-Piece Body: The Split, the Bolting, the Serviceability

The body splits at the downstream flange. The ball, both seats, the seat carriers and the thrust washers load from that side, and the joint closes on a spiral wound gasket or an O-ring and is drawn up on four or eight bolts. That single joint is the whole difference between a two-piece and a three-piece valve: one joint instead of two, so one fewer leak path at pressure, a shorter overall length, and less pipe spread needed to get the valve out for service.

On a buried installation the two-piece joint matters more than it looks. The joint sits inside the backfill, so it is the one place where a gasket failure would be invisible until the line is dug up. Specify a spiral wound gasket with a graphite filler rather than a flat sheet gasket, torque the joint bolts to the procedure, and pressure test the assembled valve rather than the components. A body joint that weeps at 20 bar in the workshop will weep faster at 20 bar in wet soil.

A105 Forged Carbon Steel: What Normalizing Buys You

ASTM A105 is a forging specification for carbon steel, not a synonym for cast steel. Carbon is capped at 0.35 percent, manganese runs 0.60 to 1.05 percent, and the material is supplied in the as forged or normalized condition. Normalizing means heating to roughly 900 C and cooling in air, which refines the grain, removes the banding that rolling leaves behind and makes hardness and toughness uniform through the section. The practical benefit at this size is dimensional: a normalized body machines without distortion, so the seat pocket stays concentric and the ball sits square on both seats.

Forging also avoids the casting quality factor that ASME B16.34 applies to cast bodies, which is one reason a forged A105 body reaches the same pressure rating with less wall thickness than a WCB casting of the same class. At DN50 the economics point the same way, because a body this small is a press forging rather than a pattern, a mould and a pour.

The limits are just as specific. A105 is qualified to -29 C, and below that the impact requirement cannot be met without changing to A350 LF2, which is a different purchase. A105 is not corrosion resistant: for wet CO2, chlorides or produced water, either add corrosion allowance or move to F316 stainless. For sour service it can be supplied to NACE MR0175 / ISO 15156 with hardness held at or below 22 HRC, but that is a documented requirement on the purchase order, not a default. And normalizing is only real if it is on the certificate: a normalized and a non-normalized body look identical after painting, so ask for the heat treatment record against the heat number.

PN20 Is a Pressure Class, Not a Flange Standard

PN20 says the valve is rated 20 bar at a 20 C reference temperature. It says nothing about how the valve bolts to the pipe, and that omission causes more flange mismatches on small European and Asian projects than any other single detail. Two valves can both be described as PN20 and still have counterflanges that will not mate, because the drilling comes from a different standard.

The bolt circle differs by four millimetres and the bolts differ in thread, so the joint cannot be made by reaming or by leaving out a bolt. The gasket seating diameters differ as well, so a gasket that fits one will not seal the other. The pressure basis also diverges: ASME derates Class 150 from 19.6 bar at 38 C as temperature rises, and EN 1092-1 rates PN20 from 20 bar at 20 C on its own curve. On a line at 180 C the two curves are not the same number, so state the design temperature and let the rating be confirmed against it rather than assumed from the class.

The ordering rule that follows is simple. Never order by pressure class alone. Order by flange standard, nominal size, pressure class and face type, in that order, and let the pressure class be the last thing written down. This valve is supplied with ASME B16.5 raised face flanges as standard and with EN 1092-1 drilling on request, and the two are not interchangeable on the same body.

The Extended Stem: Three Reasons, Three Lengths

An extended stem moves the packing gland and the operator away from the body along the stem axis. It is fitted for one of three reasons, and the length follows from the reason rather than from a preference in a catalogue. Getting the reason wrong produces a valve that is either too short to clear the cladding or too long to operate without a guide.

Insulation. On a steam or heat traced line the body sits inside 50 to 150 mm of insulation under aluminium cladding. If the gland stays at body level, either the insulation is cut away around it, which creates a cold spot, a condensate drip and a corrosion site under the cladding, or the packing runs at line temperature and the lever is buried inside the insulation. An extension equal to the insulation thickness plus about 100 mm puts the gland outside the cladding and leaves room for the lever to swing. In practice that is 150 to 250 mm for 50 to 100 mm of insulation, and 250 to 350 mm for 100 to 150 mm.

Burial. On a buried line the packing has to end up above grade, inside a valve box or a roadway sleeve. The extension length equals the depth of cover plus about 200 mm, and the box has to be sized for the lever swing or for a gear operator. At 0.8 m of cover that is a 1000 mm extension, which is no longer a stem, it is a column, and it has to be built as one.

Temperature separation. On a hot line the extension also drops the packing temperature toward ambient, which matters when the line runs close to the seat limit. It does not change the seat temperature at the ball, so an extension is not a way to raise the valve temperature rating. That is a seat material decision, covered below.

Standard lengths are 100, 150, 200, 250, 300, 400 and 500 mm. Up to 500 mm the stem is a single piece, which keeps the blow-out proof path intact and means the stem cannot be pulled out of the body if the gland is removed. Beyond 500 mm the extension is a separate tube coupled to the stem, and it then needs an anti-rotation key so the coupling cannot unwind, a thrust collar so the stem cannot be pulled upward, and a guide bearing so the assembly is not cantilevered off the gland.

What a Long Stem Changes Mechanically

An extension is not a neutral piece of tube, and three effects have to be designed for rather than discovered on site.

Bending. A hand force on a lever is 150 to 250 N at the handle. With a 500 mm extension that force acts half a metre above the gland, and the resulting moment is carried by the gland bushing and the stem rather than by the body. Any clearance in the bushing shows up as a packing leak on the far side, and repeated operation works the packing loose. A gear operator removes the side load entirely, and a guide bracket does the same thing more cheaply when the valve is buried or inside a cabinet.

Thermal growth. A 300 mm carbon steel extension heated 180 K above ambient grows about 0.68 mm, which is simply 12.5 × 10⁻⁶ × 300 × 180. If the stem and the extension are the same material the growth is shared and cancels; if the stem is 410 stainless inside an A105 tube, the differential over that length is only about 0.07 mm, which any packing absorbs. The real issue is the absolute growth compressing graphite packing and raising torque on a hot line, not the differential. A live-loaded gland, or a gland with a millimetre of travel left in it after the first hot cycle, takes care of it.

Packing and fire safe. Flexible graphite covers the full 200 C range of this valve and survives a fire; PTFE packing seals better and is the right choice for cold or clean service but is gone at 300 C. The fire safe behaviour is unchanged by the extension, because the secondary metal seat and the body joint stay where they are, but a PTFE packed extension would leave the gland as the one path that fails in a fire. If fugitive emissions are specified, the packing and gland are qualified to ISO 15848-1 and the extension length becomes part of that qualification, not an afterthought.

Seats and the Real Temperature Ceiling

This valve is often quoted as rated from -29 C to +425 C, and that range is the rating of the A105 body rather than the rating of the valve. On a soft-seated valve the seat sets the ceiling, and the two numbers are far apart. A105 in Class 150 keeps an allowable of roughly 14 bar at 400 C under ASME B16.34, so the body number is real; it just has nothing to do with what the seat can survive.

Two consequences follow. Above 250 C a metal seated floating ball valve is the correct product, and above the pressure class limits the whole design changes. And below -29 C the body is the limit, not the seat, because A105 loses its impact qualification; a cryogenic valve needs an LCB or CF8M body with an extended bonnet, which is a different build rather than an option on this one. Within its window this valve is straightforward: PTFE or RPTFE to 200 C covers water, oil, gas, and low pressure saturated steam, and PPL extends the same body to 250 C for steam and hot oil at the cost of torque.

Technical Specifications

The table is the standard envelope for this build. Values change when the seat grade, the flange standard, the extension length or the operator changes, so treat it as the baseline for a quotation rather than a fixed description.

Flow, Pressure Drop and Torque

Kv is the flow of water in cubic metres per hour that produces a pressure drop of one bar, and for this valve it is about 105, essentially the same as a full bore DN40 valve. That is the clearest way to see what reduced bore means: the ends do not set capacity, the bore does. Pressure drop follows the square of flow over Kv, so the same valve behaves very differently at 30 and at 70 m3/h.

Torque is what decides the operator, and it is not a single number. Breakaway torque rises roughly linearly with differential pressure on a soft-seated floating valve, because the seat load is produced by the pressure itself. The table below is for a PTFE seat at ambient temperature with a graphite packed stem.

Size an actuator at 1.5 times the measured breakaway torque, taken on the actual valve rather than from a catalogue, and check the figure against the minimum air supply pressure rather than the nominal one. Add about 15 percent when the extension is longer than 300 mm, because the gland bushing adds friction that a short stem does not have. A pneumatic actuator sized on nominal torque and nominal air is the most common reason a valve that turns by hand will not turn on site.

Standards and Compliance

This is a small valve, but it is specified into systems that are governed by a full set of standards, and the document package has to be able to show which one applied where.

For European projects the certificate package is to EN 10204 3.1 by default and 2.2 on request. Whether a PED CE mark applies depends on the fluid group and the PS x DN product, which for a valve this size often falls below Category I for group 2 liquids, so state the fluid and the design conditions at enquiry rather than assuming the mark is either required or absent.

Testing and Inspection

Every valve is tested as an assembly, not as a collection of parts, and the sequence below is recorded against the heat number so the report and the certificates describe the same valve.

The shell test pressurises the body and the body joint to 1.5 times the rating while the joint is watched for weeping, which is the only point in the process where a two-piece body is proved. The seat test applies 1.1 times the rating to each seat in turn with the other side open to atmosphere, and it is run in both directions because a floating valve seals on one seat at a time and the direction of installation decides which one is loaded in service. The torque record matters more than it looks on a hand operated valve: it is the number that proves the stem was not bent during assembly of the extension.

Installation, Operation and Maintenance

Check the bore and the flange faces before the valve goes near the line, and confirm the drilling against the counterflange with a tape rather than with the packing list. Make up the joint with the specified gasket, four 5/8 inch B7 bolts and a lubricated torque of about 60 to 80 Nm applied in a cross pattern, then re-check after the first thermal cycle. On a buried line, bed the body so the flange bolts carry no soil load, keep the stem within about 2 degrees of vertical, and set the valve box so the lever or the gear has full travel without touching the lid.

On an insulated line, leave the gland and the operator outside the cladding and do not insulate over the packing. A gland buried in insulation cannot be adjusted, and the first leak will soak the insulation and start corrosion under the aluminium. The extension exists precisely to avoid this, so cutting a hole in the cladding to reach a gland defeats the purchase.

Operate the valve fully open or fully closed. A floating ball valve used as a throttle erodes the downstream seat at the point where the ball edge meets the bore, and the damage is not repairable by tightening anything. For maintenance, isolate and depressurise the line, cycle the valve to confirm it is free, then split the body at the joint to inspect the ball and the seats. Replace the joint gasket every time the body is opened, check the chrome on the ball for scoring and the seat for swelling or cold flow, re-torque the joint bolts to procedure, and re-test the seat at 1.1 times the rating before the valve goes back into the line. Packing can be renewed with the valve closed and the line depressurised, but not under pressure.

Applications: Where This Build Fits, and Where It Does Not

The valve is common on insulated steam and condensate branches, heat traced chemical and bitumen lines, district heating sub-branches where the main is 2 inch and the design flow is 1.5 inch, buried yard piping and tank farm lines with a valve box, water treatment and HVAC headers, compressed air and gas skid packages, and anywhere a 2 inch connection has to be made to a 1.5 inch duty without a reducer.

The list of duties it does not fit is shorter and more useful. Each row below is a request that arrives regularly and should be redirected rather than quoted.

How to Specify and Order

A complete enquiry for this valve has eight lines, and a supplier who receives all eight can quote without a round of questions. State the end size and the bore separately, as DN50 x DN40 with the bore in millimetres. State the flange standard and drilling, ASME B16.5 Class 150 RF or EN 1092-1 PN20, and the face to face dimension if the line is already built. State the body, ball, stem and seat materials, with the seat grade named rather than left as soft seated. State the extension length in millimetres and what it is for, because that decides whether it is one piece or guided. State the operator and, if actuated, the air supply pressure and the fail position. State the design temperature and the fluid, so the rating can be confirmed against the right curve. And state the certificate, test witness and marking requirements.

Standard builds are accepted from one piece, with a regular lead time of two to four weeks for the standard seat and extension lengths and longer for a guided extension or an actuated assembly. The document package includes the EN 10204 3.1 material certificate with the normalizing record, a dimensional report, the API 598 shell and seat test report with the torque record, a coating report and a packing list. Export packing is a wooden case with the bore plugged, the flange faces protected and the extension braced so it cannot take a bending load in transit.

FAQ

Q: What does DN50x40 mean on a ball valve?

A: DN50x40 means DN50 flanged ends and a DN40 bore through the ball, so the valve drops into a 2 inch line while the flow passage is 1.5 inch, or 38 mm. The ends set the pipe size, the bore sets the capacity, the torque and whether the line can be pigged. A DN50x40 valve is functionally a 1.5 inch valve with 2 inch connections, not a 2 inch valve with a smaller hole.

Q: Is PN20 the same as Class 150?

A: For pressure, close. For flanges, no. PN20 rates 20 bar at a 20 C reference temperature under EN 1092-1; Class 150 rates 19.6 bar at 38 C under ASME B16.34. The flange drilling is different: at DN50, EN 1092-1 PN20 uses a 165 mm outside diameter, a 125 mm bolt circle and 4 x M16 bolts, while ASME B16.5 Class 150 uses 152.4 mm, 120.7 mm and 4 x 5/8 inch bolts. The two will not bolt to each other, so always state the flange standard, not just the pressure class.

Q: How long should the extended stem be?

A: Length follows the reason for the extension. On an insulated line the extension is the insulation thickness plus about 100 mm, which is typically 150 to 250 mm for 50 to 100 mm of insulation. On a buried line it is the depth of cover plus about 200 mm, so the packing sits above grade inside a valve box. Up to 500 mm the stem is one piece; beyond that the extension is a separate guided tube with an anti-rotation key and a thrust collar.

Q: Why is the temperature limit 200 C when the body is rated to 425 C?

A: Because on a soft-seated valve the seat sets the limit, not the body. ASTM A105 in ASME Class 150 retains an allowable of roughly 14 bar at 400 C, and that figure is often copied into catalogues as the valve limit. The PTFE, RPTFE or PPL seat reaches its own limit far earlier, at about 180 to 250 C depending on grade. Above that the seat deforms and the valve leaks, so the real ceiling is the seat.

Q: Can this valve be installed buried?

A: Yes, provided it is ordered with the extended stem and a valve box. The extension brings the packing gland and the operator above grade, and the body stays under the backfill. Keep the stem within about 2 degrees of vertical, bed the body so the flange bolts are not carrying soil load, and never use the extension as a pipe support or a step.

Q: Does a floating ball valve need a cavity relief valve?

A: No. In a floating ball valve the upstream seat does not seal against pressure, so the body cavity stays connected to the upstream line and nothing is trapped. Thermal expansion of the fluid in the cavity therefore cannot overpressure the body. A cavity relief connection is needed only when both seats seal against pressure, which is the double piston effect arrangement used on trunnion mounted valves.

Q: What torque does a DN50x40 PN20 floating ball valve need?

A: With a PTFE seat at the full 19.6 bar differential, breakaway torque is typically around 55 Nm, falling to about 34 Nm at 10 bar. The figure rises with differential pressure because the seat load comes from line pressure acting on the seat area. Size an actuator at 1.5 times the measured breakaway torque, against the minimum air supply rather than the nominal one, and add about 15 percent when the extension is longer than 300 mm.